1. Field of the Invention
The present invention relates to a passive component, and more particularly to a passive component suitable for use in an unbalanced to balanced converting circuit for converting an unbalanced input signal into a balanced output signal or for use in a composite circuit which includes a filter having at least one resonator and an unbalanced to balanced converting circuit.
2. Description of Related Art
Generally, there is known a balun transformer (unbalanced to balanced converter) as a circuit component for converting an unbalanced input signal into a balanced output signal or converting a balanced input signal into an unbalanced output signal.
Recently, semiconductor components such as integrated circuits (ICs) or the like have been highly integrated, while semiconductor components have quickly become smaller in their size. Accordingly, balun transformers have also become smaller in size.
One conventional balun transformer has a ½-wavelength unbalanced transmission line and a pair of ¼-wavelength balanced transmission lines (see Japanese Laid-Open Patent Publication No. 2002-299127).
The unbalanced transmission line has an end serving as an unbalanced input terminal of the balun transformer and the other end serving as an open end. Each of the balanced transmission lines has an end serving as a balanced output terminal of the balun transformer and the other end connected to ground.
Conventional passive components incorporating such a balun transformer are disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 and Japanese Laid-Open Patent Publication No. 2003-087008.
The passive component disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 is a high-frequency component incorporating a balun and a filter. The disclosed passive component includes a balun for converting a balanced line signal into an unbalanced line signal and vice versa, and a filter electrically connected to the balun, for passing or attenuating a certain frequency component. The disclosed passive component has an electrode layer which provides electrode patterns of the balun and the filter, and a dielectric layer. The electrode layer and the dielectric layer are stacked into an integral assembly.
The passive component disclosed in Japanese Laid-Open Patent Publication No. 2003-087008 includes, within a dielectric substrate, a filter section having input resonant electrodes and output resonant electrodes of two ¼-wavelength resonators, a converter having a plurality of striplines, and a connector which connects the filter section and the converter to each other.
A passive component disclosed in Japanese Laid-Open Patent Publication No. 2005-080248 is a stacked bandpass filter which is capable of outputting a balanced signal and which is small in size and can easily be adjusted. The disclosed passive component includes an unbalanced input terminal, a balanced output terminal, and a bandpass filter section connected between the unbalanced input terminal and the balanced output terminal. The bandpass filter section has a multilayer substrate on which a plurality of resonators comprising respective TEM lines are integrated. The bandpass filter section has resonators including an input resonator and a balanced-output ½-wavelength resonator which comprises a double-open-ended ½-wavelength resonator. The unbalanced input terminal is connected to the input resonator through a capacitor, and the balanced output terminal is connected to the balanced-output ½-wavelength resonator through a capacitor.
Although the passive components disclosed in Japanese Laid-Open Patent Publication No. 2004-056745 and Japanese Laid-Open Patent Publication No. 2003-087008 have the filter and the balun integrally combined with each other by the multilayer substrate or the dielectric substrate, since filter and the balun are separate circuits, the number of parts is so large that a circuit including the bandpass filter and the balun causes a large loss and has a large size.
In the stacked bandpass filter disclosed in Japanese Laid-Open Patent Publication No. 2005-080248, on the other hand, the two balanced output terminals are connected to the balanced-output ½-wavelength resonator which comprises a double-open-ended ½-wavelength resonator. Therefore, the stacked bandpass filter is capable of outputting balanced signals from the two balanced output terminals without using a balun.